1. Field of the Invention
The present invention generally relates to blood oxygen saturation probes or sensors and, in particular, to blood oxygen saturation probes or sensors which employ an optical technique for sensing the level of blood oxygen saturation.
2. Description of Related Art
The amount of saturation of blood by oxygen is a useful physiologic index of oxygen transport. A variety of techniques have been provided for measuring the level by which arterial blood is saturated by oxygen. The majority of oxygen carried in healthy blood is bound to hemoglobin, the remainder being dissolved in the plasma. Hemoglobin, the oxygen-carrying protein in blood, chemically binds to oxygen atoms and, in doing so, undergoes a change in physical structure which affects the light absorption properties of the hemoglobin.
One technique for measuring oxygen saturation in blood exploits the change in the light absorption properties of hemoglobin to detect the oxygen saturation level optically. With the optical technique, blood, or a portion of tissue carrying blood, is optically illuminated by a sensor or probe connected to an instrument, commonly referred to as an oximeter. The optical blood oxygen saturation measurement is dependent on the relative absorption of infrared light by saturated versus desaturated hemoglobin.
Thus, optical methods of blood oxygen saturation measurement rely on the measurement of transmitted or reflected light through or from a perfused tissue block with the magnitude of the transmitted or reflected light indicating the amount of oxygen bound to hemoglobin. However, the amount of reflected or transmitted light is affected by the total optical path, which is related to the size and absorption of the tissue block and the distance of emitters and sensors from the block. These factors are not controlled at a given anatomical site. Hence, a reference light source of wavelength not absorbed by saturated hemoglobin is used to obtain a baseline signal. Thus, transmitted or reflected light intensity measurements are made using two alternating light sources, one source to establish baseline, and a second source to determine the oxygen saturation.
When the sampling rate of saturation measurement is high enough, real time variation in saturation of blood as it flows through the tissue block is obtainable. The saturation varies as the oxygen is unloaded to the tissue and as fresh oxygenated blood flows into the tissue with each heartbeat. Therefore, pulse rate can be measured from the saturation signal. In addition, correlation of the saturation signal with the pulse signal aids in the determination of the saturation level that is most representative of the arterial blood oxygen saturation.
If saturation waveforms are measured at two sites in the body at different distances from the heart, the time relationship between distinguishing points on the waveforms can be correlated to blood pressure. This measurement requires a physiologically stable monitoring site with ample perfusion.
Heretofore, conventional optical oxygen saturation probes and sensors have not been designed for use in adequately physiologically stable monitoring sites. Rather, the typical optical blood saturation probe is mounted to the surface skin, such as on the chest, finger, or earlobe. Unwanted motion artifacts may appear in the measured oxygen saturation level due to alteration in the optical path and/or peripheral perfusion to the tissue block. Such an externally-mounted sensor or monitor is also susceptible to being accidentally displaced by physicians or other medical personnel working in the vicinity of the patient. Moreover, the tissue near the skin may not be adequately perfused with blood, particularly during physiological stress such as hypothermia or shock.
Other conventional blood oxygen saturation techniques include an intravascular sensor or probe. An intravascular probe is inserted into an artery, particularly the umbilical artery. Although an artery is a more stable site than the skin, the process of inserting an intravascular probe is invasive and is neither convenient for the physician nor completely safe for the patient. Further, to enable intravascular insertion, the blood oxygen saturation sensor must be of extremely small size, a constraint which increases the cost, and decreases the reliability, of the sensor.
Examples of previous oximeters and blood oxygen saturation sensors are provided in U.S. Pat. No. 4,830,014 to Goodman et al., U.S. Pat. No. 5,061,632 to Shepherd et al., and U.S. Pat. No. 4,805,623 to Jobis. The patent to Shepherd et al. discloses an oximeter with a capillary tube sensor. The patent to Goodman et al. includes a description of the deficiencies of prior art skin-mounted oximeters.
Heretofore, no optical blood oxygen saturation sensors or probes have been developed which have the safety, ease of use, and low cost of an external probe, while also being mountable or insertable to a physiologically stable monitoring site having adequate blood perfusion.